This invention relates to a calibration solution and a calibration method used when measuring ion concentration in medical fields, clinical fields and biochemical fields dealing with blood and other body fluids and ion concentration in variable partial gas pressure systems using an ion sensor or gas sensor having solid electrodes.
When measuring the pH and partial gas concentration as of blood and other body fluids as a solution under examination, use is usually made of a standard buffering solution prescribed in the JIS (Japanese Industrial Standard) standards and NBS standards (National Bureau Standard), e.g., phthalic acid salt, phosphate, etc., and measurement was done by using glass electrodes. In this case, electrode of the same kind and same structure as the inner electrode of the glass electrode is used as the reference electrode.
It is found that ion selective electrodes as well as minute solid microelectrodes and the like can be extensively utilized for medical sensors, and there are indications of practical use of pH sensors and other ion sensors in the field of clinical chemistry and artificial organ control monitors. When it is intended to apply sensors in the medical field, high accuracy measurement and temperature compensation requirement levels are higher than in the case of the general analysis, and it is difficult to meet these levels.
pH sensors and other ion sensors measure the electrode potential difference corresponding to ion activity. The ion activity is the product of the ion concentration and activity coefficient. Usually, a glass membrane electrode is used as a pH electrode, but it can not meet the definition of pH given as EQU pH=-log a.sub.H + (1)
This is so because it is impossible to obtain this value strictly due to the presence of liquid junction potential in contrast to the electrode solution section and an ion activity coefficient which is incapable of measurement. For this reason, a pH value which can be mathematically calculated from pH values of the inner solution and outer measurement solution, is defined and used, which is given as ##EQU1## where R is the gas constant, T is the absolute temperature, F is the Farady constant, E.sub.x and E.sub.s are battery electromotive forces in solutions X and S. The battery is composed of Pt; H.sub.2 solution X or S/saturated KCl solution, saturated caromel electrode.
Here, a solution with pH.sub.s is a standard solution. As the standard solution, a 0.05M potassium hydrogen phthalate solution is used, and the pH there is defined to be 3.998 (0.degree. C., 4.000) at 15.degree. C. This standard conforms to NBS and is adopted in Japan.
With a membrane electrode other than a glass membrane, the measurement of the electrode potential difference on the membrane surface is influenced by the coexistent matter in the solution under examination such as other ions than the subject of measurement, protein, and amino acids. This is so because the electrode potential difference is measured to calculate pH on the basis of the definition of equation (1).
In the measurement where the solution under examination is blood or like body fluids, a high accuracy of measurement is required despite slight ion concentration changes. Therefore, with calibration of a sensor on the basis of the usual method of measurement as noted above, the measurement errors are too large to expect a high accuracy sensor, particularly in the medical field.